It is known to provide aircraft propellers with a means of de-icing their surface, generally only the leading edge of the blades, in order to avoid problems associated with excessive ice, such as the deterioration of propeller efficiency.
Conventional propeller de-icing techniques involve installing electrical heating elements or heating pipework along the leading edge of each propeller blade behind any leading edge erosion shield.
It is often unnecessary to install such de-icing means along the entire radial length of each propeller blade since, for example, the influence of centrifugal loading may prevent the formation of ice at the blade tip regions.
In a typical electrical de-icing application an electrical generator driven by the rotating propeller supplies electrical current to the heating wires running along the blade leading edges. FIG. 1A shows such an 8-bladed propeller arrangement with no current being supplied for blade de-icing, and FIG. 1B shows the same propeller arrangement but with current supplied to all of the blades.
Since the electrical power supplied to the blades is proportional to the size and number of blades it is known to sequentially supply current to pairs or other groups of blades. For example, FIG. 1C shows the propeller arrangement of FIG. 1A with electrical current being supplied to two of the four opposing pairs of blades, while FIG. 1D shows electrical current being supplied to the remaining two pairs of blades.
This sequential supply of current may lead to ice accumulation during periods when no current is supplied to the blades but at a rate that does not lead to icing problems before the current is re-applied and de-icing is resumed.
With such a sequential de-icing arrangement it is clearly necessary to have a rotationally balanced de-icing arrangement to avoid vibration resulting from some blades being de-iced while others, whose de-icing current has been switched off, are allowed to accumulate ice.
Consequently such sequential current supply techniques can only be used with propeller assemblies having a number of blades that is divisible by another whole number. For example, for a six-bladed propeller, the de-icing current can be sequentially supplied to either two groups of three blades, or to three groups of two blades.
FIG. 2 shows the propeller assembly of FIG. 1 with two groups, each of four blades, being sequentially supplied with a de-icing current for a set period of time.
A problem with this known arrangement and technique is that if the propeller assembly comprises a prime number of blades then it is impossible to maintain rotationally balance of the propeller by selectively supplying a de-icing current to groups of the blades, as shown in FIG. 3.